The present invention relates to a process for preparing catalysts effective for the hydrodesulfurization of hydrocarbon oils. More particularly, it relates to a process for preparing catalysts effective for the hydrodesulfurization of hydrocarbon oils and which have a suitable range of fine pores and consist of alumina and the oxides of one or more metals selected from the group consisting of molybdenum, tungsten, iron, cobalt, and nickel.
Although the catalysts prepared according to the process of the present invention are applicable to any hydrocarbon oil, whether light or heavy, they show superior effectiveness in the desulfurization of heavy oils.
Heretofore catalysts for hydrodesulfurization composed predominantly of alumina and oxides of cobalt, nickel, and molybdenum have been prepared either by preparing a granular or molded .gamma.-alumina impregnated with an acidic or ammoniac aqueous solution containing a salt such as cobalt nitrate or nickel nitrate and a salt such as ammonium molybdate, or by mixing and kneading aluminum hydroxide gel with a solution of said salts or, otherwise, by coprecipitating a mixed solution consisting of a solution of a salt of aluminum and a solution of said salts by the help of alkali.
However, these processes have a drawback in that the desired dispersion of the metals becomes uneven. In the impregnation process it has been proposed in Japanese Patent Publication No. 1891/1962 to use a Werner-type complex compound of cobalt for achieving uniform penetration of the metals into particles. This process, however, involves many difficulties because of lack of simplicity in operation. In the gel-mixing process the metals are not dispersed evenly but are carried in the form of masses, and therefore are unable to exhibit sufficient activity.
In the coprecipitation process, similarly, differences in the optimum pH for precipitation of the various metal ions constituting said salts results in the production of uneven precipitates and thereby makes the dispersiveness of the metals poor. As a method of improving these drawbacks, it has been proposed in Japanese patent application No. 66,510/1968 (corresponding to U.S. patent application Ser. No. 858,174, filed 9/15/69, now abandoned) to prepare a dispersant consisting of alumina and a metal or a metal oxide by using an Al-metal-complex. Pore size distribution is also an important consideration in the choice of a catalyst for the hydrodesulfurization of residue oils. This consideration is related to the fact that a considerable proportion of metals and sulfuric components contained in hydrocarbon oils are present in asphaltene. During the progress of desulfurization, particles of asphaltene may be decomposed, thereby depositing nickel, vanadium and other metals contained therein--particularly vanadium--on the catalyst. This deposition shortens the catalyst life by blocking up fine pores on the surface thereof.
The above-mentioned methods, such as the impregnation process and the mixing of gel, can normally provide only catalysts having fine pores of about 50A in diameter. Although as described later, there are several methods of producing carriers having pores larger than 50A; they all have serious operational drawbacks. For example, particles of alumina may be combined together by the action of various factors when they are being produced.
It is reported by Morikawa et al. (Journal of the Chemical Society of Japan, Industrial Chemistry Section, 64, 898 (1961)) that Ni carried on Al.sub. 2 O.sub.3 which is used in the hydrogenation of toluene shows a strong destructive activity if it is in a form such as NiO that is close to a free state and liable to be reduced. On the other hand, it shows a high hydrogenation power and a very little destructive activity if it has been obtained by reducing slightly reducible nickel aluminate, being finely and evenly dispersed.
It is easily understood from the foregoing that the reactivity, selectivity and life of catalysts may vary remarkably with the processes and conditions by and under which they are produced.
In the process according to Japanese Patent Application No. 66,501/1968, it is possible to produce dispersants whose constituents are dispersed evenly to a higher extent than previously possible, but subject to the limitation that the peak diameter of pore size distribution is about 60A or lower. A peak diameter of about 50A offers no problem so far as the reaction with normal hydrocarbons is concerned. In the case of heavy oils, however, it has been found from the results of various tests that the diffusion of reactants is a rate-determination step in the demetalization reactant of metal-containing compounds and asphaltene contained in said oils. Thus, if the pore size of the catalyst is small, metal is liable to penetrate the surface layer of the catalyst, deposit and block up the pores.
On the other hand, with respect to the desulfurization reaction, it was found from similar tests that the condition of dispersion, kind and quantity of the metals which improve the activity of surface and more important since sufficient reaction activity can be attained if surface reaction is carried out in a rate-determining way and fine pores are larger than 50A. It was also found, however, that the devanadium reaction can be produced with more ease with an increase in the peak diameter of pore size distribution and shorten the life of the catalyst since the deposition of vanadium will proceed faster than desulfurization.
The desulfurization reaction is governed by the reaction activity of the catalyst which is, in turn, dependent upon the condition of dispersion and the manner of adhesion of the metals. On the other hand, the demetallization reaction which is considered to have a great influence on the life of the catalyst depends on the peak diameter of pore size distribution, that is, the larger the diameter becomes, the faster the reaction proceeds. From a viewpoint of the life of the catalyst, there exists an optimum range of pore diameter extending from about 50 to 200A, particularly 70 to 150A.
Carriers having fine pores larger than 50A have heretofore been prepared by adding various kinds of additives to aluminum hydroxide gel, aging aluminum gel while drying, treating aluminum gel by steam or hydrolyzing an Al-alkoxide, such as aluminum isopropoxide. All these processes have preventive steps against the consolidation of alumina particles which may be brought about by some temperature, atmospheric conditions, steam pressure or hydrophobicity at which they are produced.
As a result of an investigation of the conventional processes using an aluminum complex, it was found that the alumina obtained by simply calcinating the Al-complex is unsuitable for the purpose because of its smaller fine pores ranging in diameter from about 35 to 45A. Thus the adoption of a well-reproducible hydrothermal treatment for enlarging the particles of the aluminum complex was considered. When the diameter of fine pores in the aluminum complex thus treated was examined, however, it was found that diameters ranged broadly from 35 to 1000A and, additionally, that the pore volume was small.
It is believed that this comes from the fact that the aluminum complex itself is composed of various molecules having different molecular weights and that the reaction of hydrothermal treatment is one of hydrolyzing reactions, which do not proceed evenly when different molecules are involved.
According to the invention, a method has been discovered for controlling such hydrolyzing reactions. It has now been discovered that molybdenum compounds are effective for uniformly obtaining micro-crystals having a narrow colloidal range. Since the aluminum complex and the molybdenum compounds are positively and negatively charged, respectively, there may be some union or strong interaction between them. By adding the latter, the growth of the coagulated crystals of alumina hydrate in the aluminum complex can be prevented, thereby producing crystals of uniform size.
It is an object of the present invention, therefore, to provide a catalyst having an effective pore size for the prevention of the deposition of nickel and vanadium on the catalyst in view of the fact that such precipitation and deposition, which are considered to be responsible for the deterioration of the catalyst used in the desulfurization of hydrocarbon oils, e.g., residue oils, are largely dependent upon the size of the fine pores in the catalyst. Another object of the invention is to provide a method of preparation of catalysts that consume only a small amount of H.sub.2 ; that is, promote only desulfurization selectively without causing destructive hydrogenation.